158 THE DEVELOPMENT OF THE CHICK 
in the neural suture; it is separated from the mesenchyme in the 
region of the fore-gut by a considerable space. (We shall call 
the latter portion of mesenchyme the axial mesenchyme of the 
head, to distinguish it from the mesenchyme derived from the 
neural crest, which later hes lat- 
eral to it, and which may thus 
be known as the periaxial layer.) 
The crest may be followed ante- 
riorly to the extreme tip of the 
neural tube, and posteriorly to 
the region of the anterior intesti- 
nal portal, which hes at about the 
transverse level of the future au- 
ditory pit (cf. Fig. 91). In the 
region of the mid-brain it spreads 
( (c3> VI. V1. ae 
eee out laterally until its peripheral 
cells reach the axial mesenchyme. 
Goronowlitsch divides the pre-otic 
portion of the neural crest into pri- 
mary and secondary ganglionic crests, 
the post-otic portion being the terti- 
ary crest. According to his account 
there is a decided difference in time 
of origin of the primary and second- 
ary crests; the primary, involving the 
Fig. 91. — Diagram of the cephalic 
neural crest of a chick of about Yregion of fore- and mid-brain, aris- 
12s. (After Wilhelm His.) ing before the secondary which in- 
cludes the region of the trigeminus 
and acustico-facialis. I have not, however, found such a difference in 
my preparations. 
At the stage of 10 somites the cells of the pre-otic neural 
crest have lost their connection with the neural tube. Behind 
the optic vesicles they have spread out laterally between the 
axial mesenchyme and the ectoderm, where they form a_prac- 
tically continuous periaxial layer, distinguishable from the axial 
mesenchyme by its greater density, and hence deeper stain; 
but apparently mingling with it at the surface of contact. 
In the stages immediately following (10-20 s), the portions 
of the periaxial layer lying above the mandibular and the hyoid 
arches condense and thicken, and form strong cords extending 
